At present, the optical access network mainly adopts EPON or GPON, and both uplink and downlink work at a single wavelength, and each user transmits data by means of time division. This mechanism of allocating time slices per user on a single wavelength limits both the available bandwidth per user and the bandwidth available to the fiber itself. Introducing WDM technology into the PON system, WDM-PON, can increase the user access bandwidth by several times or even dozens of times to meet the ultimate needs of users. Therefore, WDM-PON is also considered as the solution for the next generation access network. .
Technical solutionsIn WDM-PON system, multiple different wavelengths work at the same time. Therefore, the most direct WDM-PON solution is that there are multiple light sources with different wavelengths in the OLT. Each ONU also uses a specific wavelength of light source. Each point-to-point connection is pre-designed. The wavelength is configured and working. If the number of wavelengths is larger, the more types of light sources are required, which causes serious storage problems, which is especially prominent for ONUs. Due to the serious ONU storage problem, the fixed light source solution is difficult to apply to commercial WDM-PON systems. Therefore, the use of colorless ONU has basically become the consensus of current WDM-PON related research. The technical solution based on colorless ONU is WDM-PON. The mainstream of the system. The implementation technology of the colorless ONU can be divided into three types: tunable laser, wide-spectrum light source and no light source depending on the device used.
The tunable laser uses a wavelength-tunable laser to enable the ONU to operate at different wavelengths. The tunable laser also operates at a specific wavelength, but the wavelength can be tuned by auxiliary means such as electrical tuning, temperature tuning, and mechanical tuning. The same laser can be used in the system to produce different operating wavelengths. However, tunable lasers are more complex and expensive than the lasers used in conventional PON systems, and are therefore generally not used in current WDM-PON systems.
The second solution is to place a wide-spectrum light source in the ONU. After the emitted light comes out of the ONU, it is connected to a WDM device, such as a thin film filter or AWG, to split the signal and allow only certain wavelengths to pass and transmit. Go to the OLT at the central office. Thus each of the ONUs has the same source, but since they are connected to different ports of the WDM combiner, a separate wavelength signal can be generated for each channel. The WDM-PON system using a wide-spectrum light source in the ONU is shown in Figure 1. Wide-spectrum light sources such as SLED, ASE-EDFA, and ASE-RSOA can be used. In a WDM-PON system using a broad-spectrum light source, only a very narrow portion of the light emitted by the light source is used to carry the upstream signal, while other large amounts of energy are wasted, thus requiring the light source to provide sufficient optical power. In addition, spectral segmentation causes large linear crosstalk, limits the dynamic range of the system, and requires proper selection of the passband spectral width and channel spacing of the multiplexer and demultiplexer.
Another solution is to have no light source at the ONU. All the light sources in the system are placed at the OLT, and the AWG performs spectral splitting to provide an optical signal of a specific wavelength to the ONU, and the ONU directly modulates the optical signal to generate The uplink signal is shown in Figure 2. Depending on the path of the upstream optical signal, this type of scheme is also referred to as a reflection-based colorless ONU implementation. In this implementation, the light emitted by the broad-spectrum light source is split by the AWG and supplied to the different ONUs as the upstream light source, so there is no waste of the optical signal. Wide-spectrum light sources can still be selected with amplified spontaneous emission SLED, EDFA, RSOA, etc., called seed light sources. There are various technical solutions depending on the reflective device used. The modulator used in the non-light source ONU requires low cost, can work over the entire temperature range, is not affected by polarization, has large optical bandwidth, low insertion loss, and low noise. Commonly used reflective modulators have injection-locked FP-LD, RSOA, EAM, M-ZSOA, etc. They can work in a wide spectral range, that is, the device performance is basically independent of the wavelength of the input optical signal, so that it can be used in all ONUs. The same device realizes colorlessness of the ONU.
Promote each otherWDM-PON can be regarded as the ultimate form of PON, but it is difficult to apply on a large scale in the near future. The main reasons include: lack of international standards, less investment by equipment vendors, and various devices such as chips, optical modules and broadband light source technologies. Mature, there are only a handful of equipment manufacturers worldwide that offer commercial WDM-PON systems. NoveraOpTIcs' WDM-PON system uses injection-lock-based FP-LD technology to achieve 32 wavelengths of 1.25 Gb/s per wavelength, providing 20 Gb/s of bandwidth in a single direction. Japan's Fujitsu has also developed short-term and long-term research and development projects aimed at reducing the cost of WDM-PON technology. They use RSOA as a reflection modulation device at the ONU, and introduced a transition technology between GPON and WDM-PON - HybridGPON The (HG-PON) architecture increases the capacity of the GPON downlink by a factor of seven. Korea's IP broadband access equipment provider Corecess cooperates with ETRI and also uses RSOA to build a WDM-PON platform that can transmit 16 1.25Gb/s data. In addition, Pirelli has partnered with Telecom Italia, Alcatel-Lucent and Italtel to demonstrate their CWDM-PON based FTTB/FTTC experimental network.
In terms of operators, Korea Telecom (KT) is the world's most staunch supporter of WDM-PON technology. They cooperate with LG-Nortel and Novera OpTIcs, deployed by LG-Nortel, and equipment provided by Novera OpTIcs, starting in Gwangju in 2005. The 50,000-plus, 16-wave WDM-PON experiment has reached more than 100,000 lines in the current system deployment. Although only KT has deployed a relatively large-scale WDM-PON network for operation, other operators have expressed strong interest in WDM-PON, and there are plans to select WDM-PON as a candidate technology solution for next-generation access networks. Including NTT, Verizon and some operators in Europe. UNET in the Netherlands and INS in Norway based on LG-Nortel technology, carried out WDM-PON experiments, UNET uses WDM-PON to deploy FTTB trial commercial network to provide better performance services for high-end business users.
In addition to high costs, the lack of standardization is currently the biggest obstacle to the development of WDM-PON technology. However, with the gradual development of WDM-PON related research, standards organizations have begun to consider the standardization of WDM-PON. At present, the ITU/FSAN organization has started the next-generation PON standard research after completing the standardization work of GPON, and established the NGAPON working group. In the first phase, the NGAPON working group will consider increasing the downlink bandwidth by stacking multiple wavelengths to form a so-called stacked PON in the downlink of the existing GPON, and the related work of WDM-PON will be carried out in the future, including WDM-PON and the current TDM-PON combined hybrid PON technology.
From the current situation, the next-generation PON technology after EPON and GPON will mainly develop towards 10GEPON and 10GGPON, while WDM-PON is mainly subject to cost constraints and lack of corresponding business promotion. It will not be deployed on a large scale, but due to the advanced technology of WDM-PON, as it matures, it may become a technology choice in the medium and long term.
WDM-PON is considered to be the next generation of optical access network technology with its huge bandwidth. WDM-PON can be used in a variety of applications, such as FTTx, local aggregation transmission and possible base station backhaul, to provide differentiated services for different users. However, due to the limitations of current device maturity, equipment cost and standardization, only a small number of commercial products are deployed in the experimental office. However, with the further improvement and development of the technology, WDM-PON will occupy the future access network. A place to play a bigger role.
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